A number of catheter assemblies are known for introduction of a polymeric cannula into a vein, artery or cavity of a living subject for infusion or extraction of fluids. A widely used method of inserting such a polymeric cannula is to position a steel needle (or solid stylet) within the duct of a cannula with the tip of the needle or stylet extending from the distal end of the cannula. The needle or stylet is then inserted into a living subject after which the needle or stylet is removed from the cannula, leaving the cannula behind with at least its distal end in the subject. Additional tubing can be attached to the cannula or medicaments or nutrients can be caused to flow through the cannula into the subject.
The cannulae of the prior art have generally been made of a rigid material which does not soften and/or exhibit a change in the cross-sectional area of the duct of the cannula after insertion into a vein. More recently, as set forth in commonly assigned co-pending U.S. Application Ser. No. 780,543, filed Sept. 26, 1985, now abandoned materials have been developed which can be formulated into cannulae and which, while they are rigid enough to retain their shape during insertion, also have the property of both softening and swelling to form an enlarged duct cross-section on insertion in a blood vessel due to water pickup. Also, recent publications discuss cannulae which soften on being raised to a temperature approaching body temperature.
The cannulae which form enlarged ducts can soften on formation of such enlarged ducts. This has introduced a problem in that the softened portion of the cannulae which extends outwardly from the skin of the patient can kink or be deformed, thereby cutting off flow of nutrients, medicaments and the like, into the vein. A kinking problem also exists to an extent with the prior art rigid cannulae which are generally quite thin. To date, the art has not provided an adequate solution to these problems.
The present invention is directed to overcoming one or more of the problems as set forth above.